The main rotor blades of a helicopter are subject to pitch changing movement, flapping, and lead/lag movement. Concerning the latter, it is known to provide a damper of the unequal piston-area type in a helicopter that also has blade folding, such as the Sikorsky Models CH53-D, H53-E and Seahawk SH60-B.
When the rotor is not turning, the damper functions as an actuator to position its associated blade against a lead (or lag) stop. This ensures that the blades are in a predetermined position prior to folding or, in the case of a water landing, ensures a balanced blade configuration. Fluid pressure is provided to the dampers by a pressurized oil accumulator. Hydraulic pressure acts upon both sides of the damper piston, but since the piston area is unequal from side-to-side, unequal forces are generated that cause the piston to move in the direction of the net force to position the blade.
When the rotor is turning, the forces on the piston are still unequal, but are negligible compared to the forces that are generated by the lead/lag movement of the blade. Therefore, in flight the damper functions as a damper and not as an actuator.
In the case of an equal piston-area damper, damping can be achieved simply by venting fluid from one side of the piston to the other through an orifice. However, as an unequal piston-area damper strokes, one side expels more fluid than the other can ingest--causing pumping. On the alternate stroke, one side expels less fluid than the other side can ingest--causing cavitation. Therefore, a poppet valve is provided in the damper and works under positive hydraulic pressure to accommodate the pumping action and prevent cavitation. The operation of the poppet valve in an unequal piston-area damper/positioner is known in the art and is described, for example, in U.S. Pat. No. 3,212,584 (Young, 1965) entitled DAMPER AND POSITIONER. The accumulator which is used for positioning is also used to provide the positive hydraulic pressure for damping. Even though the pressure required for positioning, typically 1000 psi, far exceeds the pressure needed for damping, the excess pressure does not interfere with the damping mode. Since the accumulator provides the positive hydraulic pressure through a common line to each blade damper, the rupture of a hydraulic line or the failure of a single seal can cause a failure of the entire damping system.